This invention relates to a developing apparatus, and particularly to a toner supply device, used in a developing apparatus, for supplying toner particles to a developer which is a mixture of carrier particles and toner particles.
A copy machine or printer utilizing electric photography has its developing unit including means of supplying toner to a developer which consists of a mixture of carrier particles and toner particles. The toner supply means operates to supply toner particles to the developer so that the toner concentration in the developer is maintained within the specified range. The toner supply means drops toner particles stored in the toner container into the developer container, or supplies toner particles to the developer magnetic brush while metering it in the recess on the supply roller surface during the transportation.
Such developing apparatuses as mentioned above are described in U.S. Pat. Nos. 4,147,127 and 4,284,702.
However, these conventional apparatuses merely place stored toner particles on the developer surface, and, in this manner, supplied toner particles weakly adhere to carrier particles in developer.
Toner particles have an extremely small specific gravity as compared with carrier particles, and therefore toner particles may float on the developer surface instead of mixing with developer. Toner particles floating on the developer do not easily sediment, and it takes a long time before supplied toner mixes with the developer and the toner concentration in the developer rises, resulting in an inferior response and in the difficulty in accurate toner concentration control. Other problems involved are an increased quantity of toner particles dispersed into the air, and a shortened life of the developer when it is stirred more powerfully with the intention of speeding up the mixing of supplied toner particles.
On the other hand, for simplicity of the developer stirring device, for example, the toner supply roller is arranged in contact with the magnetic brush, with a bias voltage being applied thereto, as disclosed in Japanese Patent Unexamined Publication (JP-A) 62-246081. In this developing device, the magnetic brush supplied with the toner particles is immediately used for developing an electrostatic latent image in a succeeding step. The magnetic brush (developer) supplied with the toner particles is to be used for development without being sufficiently stirred, and hence nonuniformity of a supplied amount of toner particles appears in the form of development irregularity. Such irregularity is easily seen in the developing device of this type as compared with the developing device of the type in which the developer supplied with the toner particles is returned to the container and then stirred therein. In order to obtain the image having no development irregularity, the toner supply device needs high control accuracy with which high uniformity of the toner supply is realized. The developing device of this type has such a problem and is not practical in use.